Buggy code:
public class EditTask { private static final String MESSAGE_INVALID_TASK = "Selected<seq2seq4repair_space>task<seq2seq4repair_space>does<seq2seq4repair_space>not<seq2seq4repair_space>exists!" ; private static final String MESSAGE_INVALID_CONTENT = "Edit<seq2seq4repair_space>task<seq2seq4repair_space>operation<seq2seq4repair_space>failed<seq2seq4repair_space>for<seq2seq4repair_space>invalid<seq2seq4repair_space>content!" ; private static final String MESSAGE_SUCCESS = "Edit<seq2seq4repair_space>task<seq2seq4repair_space>operation<seq2seq4repair_space>is<seq2seq4repair_space>successful!" ; private static final String MESSAGE_UNSUCCESS = "Edit<seq2seq4repair_space>task<seq2seq4repair_space>operation<seq2seq4repair_space>failed<seq2seq4repair_space>for<seq2seq4repair_space>end<seq2seq4repair_space>date//time<seq2seq4repair_space>error!" ; private static final String MESSAGE_TYPE_CHANGED = "Edit<seq2seq4repair_space>task<seq2seq4repair_space>operation<seq2seq4repair_space>is<seq2seq4repair_space>successful!<seq2seq4repair_space>Changed<seq2seq4repair_space>to<seq2seq4repair_space>%s<seq2seq4repair_space>task." ; private static final String PATTERN_EDIT_STARTENDDATETIME = "(\\d)(\\s)(\\bfrom\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" + "(\\s)(\\bto\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" ; private static final String PATTERN_EDIT_STARTDATETIME = "(\\d)(\\s)(\\bfrom\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" ; private static final String PATTERN_EDIT_STARTDATE = "(\\d)(\\s)(\\bfrom\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))" ; private static final String PATTERN_EDIT_STARTTIME = "(\\d)(\\s)(\\bfrom\\b)(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" ; private static final String PATTERN_EDIT_ENDTIME = "(\\d)(\\s)(\\bto\\b)(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" ; private static final String PATTERN_EDIT_ENDDATETIME = "(\\d)(\\s)(\\bto\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))(\\s)(([01]?[0-9]|2[0-3]):([0-5][0-9]))" ; private static final String PATTERN_EDIT_ENDDATE = "(\\d)(\\s)(\\bto\\b)(\\s)((19|20\\d\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12][0-9]|3[01]))" ; private static final String PATTERN_EDIT_TITLE = "(\\d)(\\s)(.*)" ; private static final String TYPE_DEADLINE = "DEADLINE" ; private static final String TYPE_FLOATING = "FLOATING" ; private static final String TYPE_TIMED = "TIMED" ; private static final String DEADLINE_TIME = "23:59" ; private Matcher EDIT_STARTENDDATETIME ; private Matcher EDIT_STARTDATETIME ; private Matcher EDIT_STARTDATE ; private Matcher EDIT_STARTTIME ; private Matcher EDIT_ENDDATETIME ; private Matcher EDIT_ENDDATE ; private Matcher EDIT_ENDTIME ; private Matcher EDIT_TITLE ; private boolean isValidLineNum = false ; private boolean isValidDT = false ; private boolean isTaskTypeChanged = false ; private String editedTaskType = "" ; private DateAndTime dtCheck = new DateAndTime ( ) ; public ResultSet execute ( String userContent , TaskListSet allLists , Validation validOp ) { } private void loadMatchers ( String userContent ) { } private String callMethods ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editTitle ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editStartEndDateTime ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editStartDateTime ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editStartDate ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editStartTime ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editEndDate ( String userContent , Validation validOp , TaskListSet allLists ) { } private boolean editEndTime ( String userContent , Validation validOp , TaskListSet allLists ) { Boolean isEdit = false ; if ( EDIT_ENDTIME . matches ( ) ) { int lineNum = Integer . parseInt ( EDIT_ENDTIME . group ( 1 ) ) ; LocalTime endTime = dtCheck . determineHour ( EDIT_ENDTIME . group ( 5 ) ) ; if ( validOp . isValidTask ( lineNum , allLists ) ) { isValidLineNum = true ; int taskID = validOp . checkForTaskID ( lineNum , allLists ) ; ArrayList < Task > taskList = allLists . getTaskList ( ) ; for ( int id = 0 ; id < ( taskList . size ( ) ) ; id ++ ) { Task tempTask = new Task ( ) ; tempTask = taskList . get ( id ) ; if ( taskID == ( tempTask . getTaskID ( ) ) ) { LocalDate endDate = tempTask . getTaskStartDate ( ) ; LocalDate startDate = tempTask . getTaskStartDate ( ) ; LocalTime startTime = tempTask . getTaskStartTime ( ) ; if ( ( tempTask . getTaskType ( ) . equals ( EditTask . TYPE_TIMED ) ) || ( tempTask . getTaskType ( ) . equals ( EditTask . TYPE_DEADLINE ) ) ) { isEdit = true ; <START_BUG> if ( ( tempTask . getTaskType ( ) . equals ( EditTask . TYPE_TIMED ) ) && ( ( tempTask . getTaskEndDate ( ) ) == null ) ) { <END_BUG> if ( dtCheck . isValidDueDT ( startDate , endDate , startTime , endTime ) ) { tempTask . setTaskEndDate ( tempTask . getTaskStartDate ( ) ) ; tempTask . setTaskEndTime ( endTime ) ; isValidDT = true ; } } else if ( tempTask . getTaskType ( ) . equals ( EditTask . TYPE_DEADLINE ) ) { tempTask . setTaskEndTime ( endTime ) ; isValidDT = true ; } } else { isEdit = false ; } } } } } return isEdit ; } private boolean editEndDateTime ( String userContent , Validation validOp , TaskListSet allLists ) { } }

Fix:
if ( ( tempTask . getTaskType ( ) . equals ( EditTask . TYPE_TIMED ) ) && ( ( tempTask . getTaskEndDate ( ) ) != null ) ) {